Our study last year showed that annexin forms Ca -promoted oligomers. We have now examined the effects of a larger variation in solution condition, in particular the dependence of the oligomerization on pH and ionic strength. We observed that the annexin monomer undergoes a significant conformational change in the absence of Ca in the pH range 6 to 8, which is a physiologically very important range. The expanded form of annexin XII at lower pH seems to be more capable of forming oligomers in a Ca -dependent manner while the compact form at higher pH shows less tendency to form oligomers. At pH 6.3, the oligomer appears to be hexamer. A time-resolved scattering study on the Ca -promoted hexamer formation was conducted. However, the rate of oligomerization appeared to be too fast to record with significant statistics, given the limited sample availability.